1. Field of the Invention
The invention relates to a method of monitoring the tension of a continuous endless drive elongate member mounted to be moved over a closed-loop path and having a slack run along which it forms undulations.
The invention applies more particularly to a mail handling machine having one or more bin carrousels whose bins are driven to move around a closed-loop path via a drive chain, e.g. an endless roller chain, or indeed a cable or a belt.
2. Discussion of the Background Art
Such a mail handling machine that more particularly has two superposed bin carrousels is known, for example, from European Patent Document EP 1 222 036 and is shown in FIG. 1.
In FIG. 1, the mail handling machine thus has a bottom conveyor or carrousel 2 made up of bins 2A mounted to be moved along a first closed-loop path, and a top conveyor or carrousel 3 made up of bins 3A mounted to be moved along a second closed-loop path that is superposed on the first path.
The two bin paths extend in planes that are substantially horizontal. The carrousel 2 serves first sorting outlets constituted by trays such as 2B disposed on a first horizontal level, and the carrousel 3 serves second sorting outlets constituted by trays such as 3B disposed on a second horizontal level situated above the first sorting outlet level.
FIG. 1 shows a small number of bins 2A, 3A and of trays 2B, 3B, but it should be understood that the bins are distributed over the entire length of the bin path. Carrousels having 200 or 350 bins are common in mail handling machines.
As shown in FIG. 1, the bins 2A of the carrousel 2 are slidably guided along their closed-loop path by a system of rails 2C. In particular, each bin 2A is provided with a structure having a plurality of bearing rollers that are mounted to move in a plurality of rails of the system of rails 2C. The roller structure is coupled to an endless drive chain (not shown in FIG. 1) which is engaged over a cog pulley 2D driven in rotation by a motor 2F and which is also engaged over a chain return and tensioning pulley 2E.
The bins 3A of the carrousel 3 are also guided by another system of rails 3C and are coupled to another endless drive chain (not shown) which is engaged over the cog wheel 3D driven in rotation by the motor 3F and over the chain return and tensioning pulley 3E.
The machine shown in FIG. 1 is fed with letters or with other flat items via one or more inlet branches such as 4 which inject the flat items into the bins 3A of the top carrousel 3 only. If flat items are for sorting to a sorting outlet served by the bottom conveyor 2, such flat items are transferred from a bin 3A of the conveyor 3 to a bin 2A of the bottom conveyor 2 via a transfer zone 5. In said transfer zone, the movements of the bins 2A and 3A of the conveyors 2 and 3 are synchronized by an electric shaft. In the transfer zone 5, the bins 2A and 3A of the two conveyors follow parallel paths that are very close together in a vertical plane so as to enable a flat item contained in a bin 3A to be transferred to a bin 2A merely by gravity.
In such a mail handling machine as described above, it is necessary for the two drive chains of the carrousels 2 and 3 to continue to have appropriate tension over time. It is therefore necessary to check frequently the tension of the chains since an over-tensioned chain might be damaged under forces that are too large, and an under-tensioned chain might give rise to the bins being damaged by them hitting one another.
Such checking is usually performed by visual inspection. For that purpose, an operator climbs onto the top portion of the casing of machine, opens one or more covers in the casing of the machine, and observes the two chains. In particular, the operator observes the extent to which chains are “weaving” or “floating”, but such inspection is approximate because it depends on the opinion of the operator.
In addition, since the inspection must be performed while the machine is operating, there exists a non-negligible risk of accident for the operator, who has to lean into the casing of the machine to come close enough to the chains while said chains are moving.
Finally, the chains are not inspected frequently enough to mitigate any risk of a chain breaking, due, for example, to the chain becoming rapidly over-tensioned.
In certain technical fields different from mail handling, systems are known for automatically checking the tension of an endless drive chain.
For example, Patent Document DE 3 927 892 discloses a device making it possible to check the tension of an endless drive chain, whereby pressure is exerted on the chain perpendicularly to the direction of movement of said chain, and values of reaction to the pressure are compared with known limit values.
Such checking is impossible on a drive chain of a bin carrousel of a postal sorting machine due to the presence of the bins coupled to the chain.
Patent Document DE 3 502 664 also discloses a device making it possible to check the tension of a drive chain by means of a plurality of force sensors associated with the pulleys over which the chain is engaged. Such an installation is very complex and costly to implement.
Patent Document U.S. Pat. No. 5,641,058 also discloses a method of performing continuous and real-time monitoring for checking the tension of a chain engaged over two cog pulleys by measuring the force exerted by hydraulic actuators for keeping the two pulleys a certain distance apart. That method is not suitable for the length of a drive chain of a bin carrousel of a postal sorting machine.